Recently, there has been data distribution for providing encrypted contents data to a user through electronic distribution using satellite broadcast or the Internet.
In the electronic distribution using satellite broadcast or the Internet, encrypted data is transmitted and received between a data transmission device and a receiving device in order to prevent leakage of the data to a third party. The contents to be electronically distributed include music, images, and works such as programs and texts.
With respect to the contents data such as music and images on which the copyright has been established, free duplication of such contents data even for the personal use may violate the copyright protection. Particularly, duplication of digital data, or so-called digital copy or digital dubbing, enables duplication of contents data without suffering deterioration of signals. Therefore, there need be some restrictions. For example, the audio digital interface prescribed by the IEC958 employs a copy generation restriction system called SCMS (serial copy management system) used in the existing CD (compact disc), DAT (digital audio tape) or MD (mini disc). This system is adapted for describing a copy control code to bits on the digital interface for prohibiting copy of the second and subsequent generations in executing copy for the first time (first generation) to a predetermined recording medium. Specifically, in the SCMS, the SCMS copy control code appended to or embedded in the “parent” contents data permits one-generation copy, and the SCMS copy control code is rewritten to prohibit copy with respect to the “child” contents data, which is obtained by sending and digitally duplicating the parent contents data at a duplication destination.
In the case where the service of contents data such as music data is provided through electronic distribution using the Internet or satellite broadcast, in general, the contents data such as music data is encrypted and a key (contents key) used for the encryption is also encrypted by another key (distribution key).
Meanwhile, in the case where one-generation digital duplication of the contents data such as music data can be carried out as in the conventional cases, the copy control code of the “parent” contents data permits one-generation copy, and the copy control code of the “child” contents data in the equipment at the duplication destination must be rewritten to prohibit copy.
In the case where the “parent” contents data are encrypted, if the equipment at the duplication destination has a function to decode the encrypted data, sending the contents data to the equipment at the duplication destination without decoding is preferred in view of the processing time and labor. However, since the SCMS copy control code embedded in the contents data and thus encrypted together with the contents data, the copy control code embedded and encrypted in the contents data is supplied to the equipment at the duplication destination without having its value changed. On the other hand, if the copy control code is to be rewritten to prohibit copy by the equipment on the supply side, then embedded in the contents data and sent to the equipment at the duplication destination, the contents data must be decrypted to rewrite the copy control code to prohibit copy and the contents data then must be encrypted again. The processing is complicated troublesome.
In general, the size of the contents key is approximately several bytes and the size of the contents data is several hundred kbytes to several Mbytes or even several Gbytes. Therefore, resetting the key takes a very long processing time.